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.TH PTHREAD_MUTEXATTR_GETPROTOCOL 3C "Feb 5, 2008"
.SH NAME
pthread_mutexattr_getprotocol, pthread_mutexattr_setprotocol \- get or set
protocol attribute of mutex attribute object
.SH SYNOPSIS
.LP
.nf
cc -mt [ \fIflag\fR... ] \fIfile\fR... -lpthread [ \fIlibrary\fR... ]
#include <pthread.h>

\fBint\fR \fBpthread_mutexattr_getprotocol\fR(
     \fBconst pthread_mutexattr_t *restrict\fR \fIattr\fR,
     \fBint *restrict\fR \fIprotocol\fR);
.fi

.LP
.nf
\fBint\fR \fBpthread_mutexattr_setprotocol\fR(\fBpthread_mutexattr_t *\fR\fIattr\fR,
     \fBint\fR \fIprotocol\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBpthread_mutexattr_setprotocol()\fR and
\fBpthread_mutexattr_getprotocol()\fR functions, respectively, set and get the
protocol attribute of a mutex attribute object pointed to by \fIattr\fR, which
was previously created by the \fBpthread_mutexattr_init()\fR function.
.sp
.LP
The \fIprotocol\fR attribute defines the protocol to be followed in utilizing
mutexes. The value of \fIprotocol\fR may be one of  \fBPTHREAD_PRIO_NONE\fR,
\fBPTHREAD_PRIO_INHERIT\fR, or \fBPTHREAD_PRIO_PROTECT\fR, which are defined by
the header <\fBpthread.h\fR>.
.sp
.LP
When a thread owns a mutex with the  \fBPTHREAD_PRIO_NONE\fR protocol
attribute, its priority and scheduling are not affected by its mutex ownership.
.sp
.LP
When a thread is blocking higher priority threads because of owning one or more
mutexes with the  \fBPTHREAD_PRIO_INHERIT\fR protocol attribute, it executes at
the higher of its priority or the priority of the highest priority thread
waiting on any of the mutexes owned by this thread and initialized with this
protocol.
.sp
.LP
When a thread owns one or more mutexes initialized with the
\fBPTHREAD_PRIO_PROTECT\fR protocol, it executes at the higher of its priority
or the highest of the priority ceilings of all the mutexes owned by this thread
and initialized with this attribute, regardless of whether other threads are
blocked on any of these mutexes.
.sp
.LP
While a thread is holding a mutex that has been initialized with the
\fBPRIO_INHERIT\fR or  \fBPRIO_PROTECT\fR protocol attributes, it will not be
subject to being moved to the tail of the scheduling queue at its priority in
the event that its original priority is changed, such as by a call to
\fBsched_setparam()\fR. Likewise, when a thread unlocks a mutex that has been
initialized with the \fBPRIO_INHERIT\fR or \fBPRIO_PROTECT\fR protocol
attributes, it will not be subject to being moved to the tail of the scheduling
queue at its priority in the event that its original priority is changed.
.sp
.LP
If a thread simultaneously owns several mutexes initialized with different
protocols, it will execute at the highest of the priorities that it would have
obtained by each of these protocols.
.sp
.LP
If a thread makes a call to \fBpthread_mutex_lock()\fR for a mutex that was
initialized with the protocol attribute \fBPTHREAD_PRIO_INHERIT\fR, and if the
calling thread becomes blocked because the mutex is owned by another thread,
then the owner thread inherits the priority level of the  calling thread for as
long as it continues to own the mutex. The implementation updates its execution
priority to the maximum of its assigned priority and all its inherited
priorities. Furthermore, if this owner thread becomes blocked on another mutex,
the same priority inheritance effect will be propagated to the other owner
thread, in a recursive manner.
.sp
.LP
A thread that uses mutexes initialized with the \fBPTHREAD_PRIO_INHERIT\fR or
\fBPTHREAD_PRIO_PROTECT\fR \fIprotocol\fR attribute values should have its
scheduling policy equal to \fBSCHED_FIFO or SCHED_RR\fR (see
\fBpthread_attr_getschedparam\fR(3C) and \fBpthread_getschedparam\fR(3C)).
.sp
.LP
If a thread with scheduling policy equal to \fBSCHED_OTHER\fR uses a mutex
initialized with the \fBPTHREAD_PRIO_INHERIT\fR or \fBPTHREAD_PRIO_PROTECT\fR
\fIprotocol\fR attribute value, the effect on the thread's scheduling and
priority is unspecified.
.sp
.LP
The \fB_POSIX_THREAD_PRIO_INHERIT\fR and \fB_POSIX_THREAD_PRIO_PROTECT\fR
options are designed to provide features to solve priority inversion due to
mutexes. A priority inheritance or priority ceiling mutex is designed to
minimize the dispatch latency of a high priority thread when a low priority
thread is holding a mutex required by the high priority thread.  This is a
specific need for the realtime application domain.
.sp
.LP
Threads created by realtime applications need to be such that their priorities
can influence their access to system resources (\fBCPU\fR resources, at least),
in competition with all threads running on the system.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, the \fBpthread_mutexattr_getprotocol()\fR and
\fBpthread_mutexattr_setprotocol()\fR functions return  \fB0\fR. Otherwise, an
error number is returned to indicate the error.
.SH ERRORS
.sp
.LP
The  \fBpthread_mutexattr_getprotocol()\fR and
\fBpthread_mutexattr_setprotocol()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 11n
The value specified by \fIattr\fR is \fINULL\fR.
.RE

.sp
.ne 2
.na
\fB\fBENOSYS\fR\fR
.ad
.RS 11n
Neither of the options \fB_POSIX_THREAD_PRIO_PROTECT\fR and
\fB_POSIX_THREAD_PRIO_INHERIT\fR is defined and the system does not support the
function.
.RE

.sp
.ne 2
.na
\fB\fBENOTSUP\fR\fR
.ad
.RS 11n
The value specified by \fIprotocol\fR is an unsupported value.
.RE

.sp
.LP
The \fBpthread_mutexattr_getprotocol()\fR and
\fBpthread_mutexattr_setprotocol()\fR functions may fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The value specified by \fIattr\fR or \fIprotocol\fR is invalid.
.RE

.sp
.ne 2
.na
\fB\fBEPERM\fR\fR
.ad
.RS 10n
The caller does not have the privilege to perform the operation.
.RE

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed
_
MT-Level	MT-Safe
_
Standard	See \fBstandards\fR(7).
.TE

.SH SEE ALSO
.sp
.LP
.BR pthread_attr_getschedparam (3C),
.BR pthread_mutex_init (3C),
.BR pthread_mutexattr_init (3C),
.BR sched_setparam (3C),
.BR sched_setscheduler (3C),
.BR attributes (7),
.BR standards (7)
